Multiple-Symbol Detection for Photon-Counting Free-Space Optical Communications

Free-space optics (FSO) have received increased attention recently for last-mile, line-of-sight wireless links. In this paper, we employ a photon-counting signal model of an FSO system and investigate noncoherent detection, i.e. detection assuming the absence of channel state information at the receiver, of on-off keying (OOK). To partially recover the performance loss associated with symbol-by-symbol noncoherent detection, we consider the application of multiple-symbol detection (MSD), in which block-wise decisions are made using an observation window of N bit intervals. We develop a fast search algorithm for optimal MSD, which corresponds to a fixed bit-wise implementation complexity effectively independent of N. Furthermore, we obtain performance expressions for an ideal decision-feedback receiver, which serve as an upper bound to the performance of the MSD receiver. Analytical and simulation results indicate that the performance of the MSD receiver approaches the coherent detection lower bound with increasing N.